1. Field of the Invention
The present invention relates to an electric motor control unit for controlling a rotational speed of a motor used for a cooling fan of a radiator mounted on an automotive vehicle, particular to a motor control unit for controlling the rotational speed of the motor based on detection of overcurrent supplied to the motor.
2. Description of the Related Art
Conventionally, for overcurrent protection of a cooling fan of a radiator mounted on an automotive vehicle, fuses have been generally used. This causes various problems, which will be described hereinafter with reference to the accompanying drawings.
FIG. 8 is a circuit diagram showing an example of a conventional motor control unit. The motor control unit is assembled in an electric junction box incorporated with a function circuit. The electric junction box is called as a junction block that is mounted on many recent automotive vehicles.
An electric junction box 8 shown in FIG. 8 has a power input terminal 81 and an output terminal 82 which are electrically connected respectively to a battery 2 or a motor 9. The battery 2 is, for example, a known 12-volt battery, and the motor 9 is used for driving an engine cooling fan. The motor 9 has, for example, a power terminal 9a and a ground terminal 9b. 
The electric junction box 8 accommodates a microcomputer 86 for receiving external control signals via a signal input terminal 83 and for outputting signals to a transistor 85 to open and close a relay 84. The relay 84 closes a contact 84b on excitation of a coil 84a due to a constant current supplied from the battery 2 to deliver an electrical power to the motor 9. Between the battery 2 and the power input terminal 81, there is provided a fuse f.
In this configuration, for example, the microcomputer 86 receives an external control signal via the signal input terminal 83 to start the motor 9, so that the microcomputer 86 outputs a high level signal to turn the transistor 85 on. Thereby, electrical current flows from the battery 2 to the coil 84a of the relay 84 via the power input terminal 81 to close the contact 84b. Accordingly, the motor 9 supplies an electrical power to the power terminal 9a of the motor 9 via the contact 84b of the relay 84 and the output terminal 82. This operates the motor 9. On the contrary, the microcomputer 86 receives an external control signal via the signal input terminal 83 to stop the motor 9, so that the motor 9 is brought into its stopped state.
During an operating state of the motor 9, overcurrent generally blows the fuse f for preventing damage of circuit parts. However, since the fuse has a variation in a fusing performance thereof, the fuse will not possibly be blown at a predetermined overcurrent for the relay coil. This will possibly cause damage of the coil due to the overcurrent, decreasing usage lives of the circuit parts.
Even when the fuse f is blown correctly, another problem occurs as described hereunder. For example, during an operating state of the vehicle to run through a flooded road, the cooling fan driven by the motor 9 may be completely or almost locked due to water immersion. The motor 9 tends to continue the rotation against a resistance force of the water. This generates overcurrent called as a lock current, which blows the fuse f to stop the motor 9. A continued operation of the vehicle with the fuse f having been blown may cause an undesirably high water temperature of the radiator when the vehicle has run through the flooded road but a driver fails to recognize the stopped state of the motor. Accordingly, the provision of the overcurrent protection fuse of the conventional art includes the aforementioned problem.